1. Field of the Invention
The present invention relates to an image forming apparatus comprising a first optical means for projecting a light image of an original to be copied on an electrophotographic photosensitive member, and a second optical means for illuminating light modulated in correspondence to an image signal onto the electrophotographic photosensitive member.
2. Related Background Art
There has been proposed a copying apparatus comprising an analogue exposure means (first optical means) for directly projecting an original image on a photosensitive member by means of an optical system including a light source such as a halogen lamp or a fluorescent lamp, reflection mirrors and optical lenses, and a digital exposure means (second optical means) for illuminating light onto the photosensitive member in response to a digital image signal by using a laser or an LED array as a light source. This copying apparatus was so designed that the digital exposure means removes the charge in a non-image area other than an original image (first image) area formed by the analogue exposure means and writes an additional information image (second image) on the first image.
The above digital exposure means can be utilized to copy color information defined within an area of the same or different original image encircled by a mark and the like, or to copy characters, figures, date or the like stored in the copying apparatus itself, or to copy information inputted from external equipments other than the copying apparatus through the communication. That is to say, when the image is written by the digital exposure means, the analogue exposure is partly shielded, and the shielded area is exposed by the modulated light by means of the digital exposure means with the background scan (scanning operation wherein the latent image charge in the non-image area is removed with remaining the latent image charge in the image area) to form an electrostatic latent image, and then the latent image is developed by a developing means to obtain a visualized image.
By the way, the sensitivity of the electrophotographic photosensitive member is varied by repeating the image forming operations. That is to say, for a long period of time, the potential (Vd) of a dark portion of the photosensitive member is decreased and the potential (V1) of a bright portion is increased. Further, during the continuous copying operation and the like, the potentials of the dark and bright portions are both reduced temporarily. For these reasons, the reduction in density of the copy image and(or) the fog of the copy image occur.
In order to solve these problems, an image forming apparatus may be so designed that the charging potential and(or) the exposure quantity are adjusted on the basis of an output signal from a sensor provided for measuring a surface potential of the photosensitive member. However, the apparatus having such potential measuring sensor requires the exclusive complicated control circuit, thus making the apparatus expensive.
Of course, the image forming processes may be manually adjusted individually without the provision of such control circuit; in this case, however, the adjusting operation will be complicated and troublesome.